Diabetes mellitus is a disorder in which the pancreas cannot create sufficient insulin (Type I or insulin dependent) and/or in which insulin is not effective (Type 2 or non-insulin dependent). In the diabetic state, the victim suffers from high blood sugar, which can cause an array of physiological derangements associated with the deterioration of small blood vessels, for example, kidney failure, skin ulcers, or bleeding into the vitreous of the eye. A hypoglycemic reaction (low blood sugar) can be induced by an inadvertent overdose of insulin, or after a normal dose of insulin or glucose-lowering agent accompanied by extraordinary exercise or insufficient food intake.
Conventionally, a person admitted to a hospital for certain conditions (with or without diabetes) is tested for blood sugar level by a single point blood glucose meter, which typically requires uncomfortable finger pricking methods or blood draws and can produce a burden on the hospital staff during a patient's hospital stay. Due to the lack of convenience, blood sugar glucose levels are generally measured as little as once per day or up to once per hour. Unfortunately, such time intervals are so far spread apart that hyperglycemic or hypoglycemic conditions unknowingly occur, incurring dangerous side effects. It is not only unlikely that a single point value will not catch some hyperglycemic or hypoglycemic conditions, it is also likely that the trend (direction) of the blood glucose value is unknown based on conventional methods. This inhibits the ability to make educated insulin therapy decisions.
A variety of sensors are known that use an electrochemical cell to provide output signals by which the presence or absence of an analyte, such as glucose, in a sample can be determined. For example, in an electrochemical cell, an analyte (or a species derived from it) that is electro-active generates a detectable signal at an electrode, and this signal can be used to detect or measure the presence and/or amount within a biological sample. In some conventional sensors, an enzyme is provided that reacts with the analyte to be measured, and the byproduct of the reaction is qualified or quantified at the electrode. An enzyme has the advantage that it can be very specific to an analyte and also, when the analyte itself is not sufficiently electro-active, can be used to interact with the analyte to generate another species which is electro-active and to which the sensor can produce a desired output. Such conventional sensors can employ a silver/silver chloride reference electrode. Over time, such a reference electrode becomes depleted, as silver ion is converted to silver metal. As silver ion is depleted, reference electrode capacity decreases, reducing the stability of the reference electrode such that glucose sensor becomes less linear.